1. Field of the Invention
Embodiments of the present invention generally relate to field effect transistors (FETs) and thin film transistors (TFTs) having semiconductor materials comprising oxygen, nitrogen, and one or more elements selected from the group consisting of zinc, gallium, cadmium, indium, and tin.
2. Description of the Related Art
Current interest in TFT arrays is particularly high because these devices may be used in liquid crystal active matrix displays (LCDs) of the kind often employed for computer and television flat panels. The LCDs may also contain light emitting diodes (LEDs) for back lighting. Further, organic light emitting diodes (OLEDs) have been used for active matrix displays, and these OLEDs require TFTs for addressing the activity of the displays.
TFTs made with amorphous silicon have become the key components of the flat panel display industry. Unfortunately amorphous silicon does have its limitations such as low mobility. The mobility required for OLEDs is at least 10 times higher than that achievable with amorphous silicon. The deposition temperature for amorphous silicon may be high, which can cause a Vth shift. A high current may be necessary for amorphous silicon, which can lead to stability issues in OLEDs. Polysilicon, on the other hand, has a higher mobility than amorphous silicon. Polysilicon is crystalline, which leads to non-uniform deposition. Due to the limitations of amorphous silicon, OLED advancement has been difficult.
In recent years, transparent TFTs have been created in which zinc oxide has been used as the active channel layer. Zinc oxide is a compound semiconductor that can be grown as a crystalline material at relatively low deposition temperatures on various substrates such as glass and plastic. Zinc oxide based TFTs may not degrade upon exposure to visible light. Therefore, a shield layer, as is necessary for silicon based TFTs, is not present. Without the shield layer, the TFT remains transparent. Zinc oxide, while having a mobility greater than amorphous silicon, still has a low mobility.
Therefore, there is a need in the art for TFTs having transparent active channels with high mobility.